1. Field of the Invention
The present invention relates to an optical semiconductor module obtained by mounting an optical semiconductor device such as a semiconductor laser on a modular substrate for an optical coupling and, more particularly, to an optical semiconductor module in which a coupling mechanism between optical semiconductor devices and optical fibers, a hermetically sealed capping structure, and a connecting mechanism using guide pins are improved.
2. Description of the Related Art
Semiconductor devices are generally subjected to hermetic sealing or plastic molding to prevent its degradation caused by humidity, dust, and ions in the atmospheric air. In particular, since optical semiconductor devices have optical input/output surfaces which can be easily contaminated, a normal package has a hermetic sealing structure. Hermetic sealing employs a method of mounting a metal or ceramic substrate having a semiconductor device on a package consisting of a metal or the like and having terminals insulated with glass, and mounting a cap consisting of a metal or the like on the package by soldering, welding, or the like.
A hermetic sealing package is shown in FIG. 1. Referring to FIG. 1, reference numeral 1 denotes an optical semiconductor device. This optical semiconductor device 1 is mounted on an Si submount 2 mounted on a metal stem 5. The Si submount 2 is used to prevent distortion caused by a difference between the thermal expansion coefficients of the metal stem and the optical semiconductor devices. Reference numeral 3 denotes each metal lead sealed to be insulated from the metal stem 5 by glass 4.
The optical semiconductor device 1 is electrically connected to the metal leads 3 by bonding wires 7. Reference numeral 6 denotes a metal cap; and 8, an external light incident/exit window. The window 8 is hermetically and integrally bonded to the metal cap 6. The metal cap 6 is welded to the metal stem 5, and the optical semiconductor device 1 is hermetically sealed by the above members.
When an optical semiconductor module is used as an optical transmitter for communication or information processing systems, fiber coupling is generally used as the signal transmission medium. In this manner, when the optical semiconductor module is to be coupled to an optical fiber, the following method is employed. That is, the optical fiber is fixed to a fiber holder such as a sleeve, the optical axes of an optical semiconductor device and the optical fiber are adjusted to each other, and a fiber holder is fixed to the package by laser welding or the like. As a consequence, the size of the entire optical semiconductor module becomes very large compared with the optical fiber and the optical semiconductor device chip.
Therefore, a device obtained by of fixing an optical semiconductor device and an optical fiber in a metal package and hermetically sealing them at once is known as an optical semiconductor module including an optical coupling system therein. FIG. 2 shows this optical semiconductor module. Referring to FIG. 2, reference numeral 11 denotes an optical semiconductor device which is mounted on an Si submount 12. Reference numeral 15 denotes an optical fiber which is fixed on a sleeve 16. Reference numeral 13 denotes a metal package including a structure having a base 14, 17 on which the Si submount 12 and the optical fiber 15 can be fixed.
After the optical axis of the optical fiber 15 is aligned with that of the optical semiconductor device 11, the optical fiber 15 is fixed on a base 17 with an adhesive 18 such as an ultraviolet-curing adhesive. The sleeve 16 and a package 13 are hermetically bonded to each other by welding using a solder. Reference numeral 19 denotes a lid hermetically bonded to the package 13 by welding or the like. Reference numeral 20 denotes each hermetic terminal insulated from the package 13 by glass to electrically connect the optical semiconductor device to an external circuit. In this case, since the optical semiconductor device and the optical fiber are sealed at once, this optical semiconductor module can be smaller than the above-mentioned optical semiconductor module shown in FIG. 1 in overall size. However, since the hermetic terminals and the sleeve are required, a reduction in size of the device is limited.
In addition, when an optical semiconductor device is to be used, an electric circuit for driving the optical semiconductor device must be connected outside the device. An example wherein the external electric circuit is connected to the device will be described below. A method in which several semiconductor device chips are mounted on one substrate and packaged at once is known. In particular, a multi-chip mounting method in which devices are mounted as bare chips on a substrate and hermetically sealed at once is advantageous to integration. However, in the module using optical semiconductor devices, the optical semiconductor devices form an optical submodule using an independent package hermetically sealed, and the optical submodule is connected to an external electric circuit and then sealed at once. Therefore, an optical semiconductor device is often packaged in a double structure.
In this case, since the optical semiconductor device is packaged by a metal, an Si submount is required to prevent distortion caused by the difference between the thermal expansion coefficients of the optical semiconductor device and the metal. Therefore, the optical semiconductor device is mounted on the Si submount and then hermetically sealed. For this reason, the package is larger than an electronic semiconductor device in size, and the size of the whole module is considerably increased by using the optical semiconductor device.
On the other hand, an optical semiconductor module in which an optical semiconductor device such as a semiconductor laser or a photodetector is optically coupled to an optical fiber is much more expensive than a normal semiconductor module such as a transistor or an integrated circuit. This is not because of the manufacturing cost of the optical semiconductor device, but mainly because of the coupling adjustment cost between optical semiconductor devices and optical fibers. A reduction in coupling adjustment cost is considerably difficult.
Therefore, an optical semiconductor module which can be mass-produced at low cost has been demanded, and several attempts have been reported or proposed. 0f these attempts, especially, an optical semiconductor module obtained by applying a so-called micromachining technique using the semiconductor manufacturing technique has received a great deal of attention. The characteristic feature of the optical semiconductor module obtained by applying the micromachining technique lies in that mechanical processing on the order of submicron, i.e., 1 .mu.m or less, developed by the semiconductor device manufacturing technique makes it possible to perform optical axis adjustment between optical semiconductor devices and optical fibers by only a mechanical assembly process, and a large number of optical semiconductor devices can be manufactured at once like normal semiconductor devices.
FIG. 3A is a perspective view showing a conventional optical semiconductor module as one of massproduced optical semiconductor modules free from optical axis adjustment by applying the above micromachining technique. Reference numeral 31 denotes a modular substrate in which a guide groove 32 for adjusting the position of an optical fiber is formed and which has optical semiconductor device mounting electrodes 36. Reference numeral 34 denotes an optical fiber; and 35, an optical semiconductor device, e.g., an edge emission type semiconductor laser.
In this case, the module substrate 31 consists of Si crystal as its material. The guide groove 32 is formed in the module substrate 31 using photolithography and a chemical etching technique which are the same as those of the process of manufacturing a semiconductor device, and the electrodes 36 are similarly formed in the module substrate 31 using photolithography and a evaporate deposition technique. At this time, when so-called solder bumps are used as the electrodes 36, the optical semiconductor device 35 can be precisely positioned in a self-aligned manner by the well-known function of a surface tension of the solder bumps.
On the other hand, since the optical fiber 34 is mechanically fitted in the groove 32, the optical fiber 34 can be precisely positioned. In this manner, the optical semiconductor device 35 and the optical fiber 34 which are positionally adjusted are mounted on the modular substrate 31, and the resultant structure is arranged in the outer case of a module, thereby completing an optical semiconductor module.
FIG. 3B is a sectional view showing the optical semiconductor module shown in FIG. 3A along its optical axis. Reference numeral 37 denotes the core of the optical fiber, i.e., an optical waveguide portion. The optical semiconductor device 35 has electrodes each having the same shape as that of each of the electrodes 36, the electrode surface of the optical semiconductor device 35 faces down. The electrodes are electrically connected to the electrodes 36, and at the same time, as described above, the optical semiconductor device 35 is mechanically adjusted to a predetermined position of the modular substrate 31 by the surface tension of the solder material in the process of melting the electrodes 36. At this time, the thickness of each of the electrodes 36 and the shape and depth of the guide groove 32 of the optical fiber 34 are determined in advance such that the core 37 of the optical fiber is matched with the active portion of the optical semiconductor device 35.
In the optical semiconductor module manufactured as described above, the precision of machining and mounting can be set to be 1 .mu.m or less. The optical semiconductor module can be theoretically manufactured by only a mechanical assembly process. All important technical points of the process are the application of the semiconductor technique device manufacturing, necessary parts has a high processing precision, and a large number of parts are manufactured at once, i.e., on a large number of semiconductor wafers at once. Therefore, the optical semiconductor device requires no optical axis adjustment process, and an automatic mechanical assembly process allows mass production and a great reduction in cost.
In an optical semiconductor module of this type, although mass production and the reduction in cost can be achieved as compared with the conventional optical semiconductor module requiring optical axis adjustment, the reproducibility of temperature characteristics and optical coupling characteristics poses problems because incompleteness of a self-aligning mechanism is left in control for mounting the optical fiber at a position along the optical axis, the optical axis is changed by the thermal expansion of the members caused by a change in temperature, and the like. These causes will be described below.
The module substrate 31 shown in FIGS. 3A and 3B uses anisotropic etching of Si to control the shape and depth of the guide groove 32 of the optical fiber 34 such that the shape and size of the guide groove 32 of the optical fiber 34 are automatically determined by the precision of photolithography. The anisotropic etching of Si crystal utilizes a sharp decrease in etching rate on the (111) plane, and can form various geometrical shapes using an etching-initiating crystal face and the shapes of a mask. A combination for uniquely determining the width and depth of the guide groove is a combination for etching a substrate aligning a slit-like mask on the (001) plane in the &lt;110&gt; or &lt;1-10&gt; direction. In this case, a V-groove is formed in the &lt;110&gt; or &lt;1-10&gt; direction, and the depth of the groove can be determined by the width of the mask.
However, an end portion of the guide groove 32, i.e., the terminal end portion of the guide groove 32 for receiving the optical fiber 34 has no vertical wall but an inclined wall having an inclination angle as that of the V-groove. For this reason, the lower portion of the end face of the optical fiber 34 is brought into contact with a terminal end portion 38 of the V-groove, thereby determining the position of the optical fiber 34 along the optical axis. However, this causes the degradation of reproducibility of temperature characteristics and optical coupling characteristics. FIG. 4A shows this state. The end portion of the optical fiber 34 is bent by the inclined surface of the terminal end portion 38 of the guide groove to decenter the optical axis (broken line 34').
When the optical fiber 34 is bent as indicated by reference numeral 34' in FIG. 4A, the core 37 of the optical fiber is easily decentered by several .mu.m, or 10 .mu.m or more depending conditions, and optical coupling is considerably degraded. This bending of the optical fiber occurs when an excessive axial pressure is applied to the optical fiber 34 during assembling the module, or when the difference between the thermal expansion amounts of the optical fiber 34 and the module substrate 31 is increased with a change in ambient temperature after the module is assembled. In addition, the terminal end portion 38 of the guide groove may be spatially separated from the end portion of the optical fiber 34 to solve the above problem. However, in this case, the positional relationship between the optical fiber 34 and the optical semiconductor device cannot be precisely controlled in the optical axis, and the reproducibility of light coupling characteristics cannot be assured.
As described above, in the prior art shown in FIGS. 3A and 3B, although an optical semiconductor module can be theoretically manufactured at low cost in mass production, the optical semiconductor module still has structural or mechanical drawbacks. In addition, these drawbacks are more conspicuous when a plurality of optical fibers are arranged than when a single optical fiber and an optical semiconductor device are used. This is because, when a plurality of optical fibers are used as an optical fiber array, i.e., a so-called ribbon fiber, and the distal end positions of the optical fibers are shifted from each other due to the manufacturing characteristics of the ribbon fiber. FIG. 4B shows this state. Four optical fibers 34a to 34d are banded together as a ribbon fiber by a jacket 39.
In general, this ribbon fiber is formed such that independent optical fibers drawn from different preforms are banded by an ultraviolet-curing resin or the like to form an array. For this reason, the elastic characteristics of the optical fibers are slightly different from each other, and different stresses are applied to the optical fibers by twisting the optical fibers during banding. Therefore, even when the resin of the ribbon fiber is partially removed, and the optical fibers are cut to have a uniform length, the positions of the distal end portions of the optical fibers are changed over time to easily cause variations in length. In addition, even the changes in positions of the distal end portions can be suppressed as small as possible by a careful operation, the positions of the distal end portions are easily changed by heating and cooling operations in assembling the module.
In FIG. 4B, a line A-A' represents a position where the distal end portion is most retreated, and a line B-B' represents a position where the distal end portion is most advanced. The lines A-A' and B-B' are easily shifted by several tens .mu.m. For this reason, when a plurality of optical fibers are used, the problem shown in FIG. 4 is more conspicuously posed.
As described above, in an optical transmission module, an optical switch module, or the like used in optical communication, an optical semiconductor device such as a semiconductor laser or an optical waveguide must be optically coupled to optical fibers. For this purpose, many methods of adjusting optical axes and methods of fixing optical fibers have been proposed.
Most of the conventional methods of adjusting optical axes and fixing optical fibers are as follows. That is, optical axis adjustment is performed by actually transmitting light through an optical fiber so as to obtain a maximum optical coupling value, and respective parts are fixed by some means. Although these methods are used in the process of manufacturing actual products, optical axis adjustment requires skills and a long period of time because fine positional adjustment must be performed. In addition, since optical axes are changed by distortions or stresses during fixing parts, even optical axis adjustment for a long period of time inevitably result in defective products. That is, the manufacturing cost itself is high due to a long manufacturing time, and a loss caused by defective products is higher than a material cost. The manufacturing cost is further increased accordingly.
The method of performing the optical monitoring poses another problem as follows. That is, it is more difficult to manufacture an array of optical fibers, and the yield is decreased by a power of the number of optical fibers in an array compared with the yield of single optical fibers. For this reason, the manufacturing cost considerably is increased, and products may difficult to manufacture depending on the number of optical fibers in an array.
On the other hand, a method of mounting an optical fiber without using optical monitoring is proposed. For example, a method of mounting an optical fiber by a mechanical fitting operation and adjusting the position of the optical fiber with mechanical dimensional precision. According to this method, the optical fiber can be mounted and assembled by mechanically assembling parts. In addition, since a plurality of optical fibers can be basically mounted at once, the number of manufacturing steps and a period of time required for manufacturing the module can be decreased compared with those of a method in which optical monitoring is performed, and losses caused by defective products are almost determined depending on a material cost. This method is especially effective for mounting an optical fiber array. A prior art according to this method is shown in FIG. 5.
Referring to FIG. 5, reference numeral 41 denotes a mounting holder; and 43, optical fibers 43. The mounting holder 41 is formed such that anisotropic chemical etching is performed to, e.g., an Si substrate, to form V-grooves. At this time, when a mask for chemical etching is formed by the same photolithography as that used in the semiconductor process, processing can be performed at a precision of about 1 .mu.m, and a plurality of mounting holders 41 can be formed at once. According to this method, optical fibers can be highly precisely mounted in principle. For example, a method of mounting the optical fibers is performed as shown in FIG. 6.
Referring to FIG. 6, reference numeral 48 denotes an adhesive material for mounting the optical fibers. For example, an ultraviolet-curing resin is used as the adhesive material 48. Reference numeral 49 denotes a press plate for mounting the optical fibers. A flat Si substrate or the like is used as the press plate to uniformly press the optical fibers 43 against the holder 41. The positions of the optical fibers 43 pressed against the mounting holder 41 by this method have been mechanically adjusted, and, in particular, the arrangement precision (mounting pitch) of the optical fibers 43 are very high.
However, in the method of mounting optical fibers by a mechanical fitting operation, the arrangement precision, i.e., a relative positional precision, of the optical fibers is actually high, but an absolute positional precision of the optical fibers is not always assured.
In a conventional optical fiber mounting body, optical fibers are positioned by bringing the optical fibers into contact with the grooves of a mounting holder. For this reason, the conventional mounting holder must have a fine surface state, and a method of processing the mounting holder is almost limited. A conventional jig for mounting the optical fibers has only a function of pressing them against the mounting holder, as shown in FIG. 6. Although this jig can press the optical fibers against the grooves of the mounting holder, positions at which the optical fibers are mounted in the grooves cannot be controlled. That is, the jig is passive with respect to the groove processing precision of the mounting holder, and any correcting means cannot be applied to the mounting jig for the precision of the depth of each groove. The precision of the depth cannot be corrected regardless of changes in external pressure and pressure application angle.
FIG. 7 is a sectional view showing a state wherein the optical fibers 43 are fitted in the grooves of a mounting holder 41' in which the grooves deeper than the grooves of the mounting holder 41 shown in FIG. 6 are formed. Referring to FIG. 7, when the deep grooves are formed, the positions of the fitted optical fibers 43 are lower than those shown in FIG. 6.
In addition, when a fixing material for the optical fibers is brought into contact with the surface of the mounting jig, the mounting jig itself may be fixed. For this reason, an amount of fixing material must be kept proper, and when the fixing material is accidentally brought into contact with the mounting jig, a manufacturing apparatus must be stopped, and the mounting jig must be replaced and adjusted again.
The grooves are formed in the mounting holder by performing anisotropic etching on a surface having a predetermined crystal orientation. In this case, since this etching progresses along the crystal orientation, as shown in FIG. 8, when the angle of an etching mask 51 is shifted from a crystal orientation 52 of the mounding holder indicated by broken lines, the surface of the mounting holder is etched along the crystal orientation regardless of the opening of the etching mask 51. As a result, the width and positions of the grooves are shifted with respect to those of the etching mask.
On the other hand, although the mounting conditions of optical fibers in the same mounting holder are equal to each other by simultaneous mounting, the mounting conditions of a plurality of mounting holders are not necessarily equal to each other. The mounting conditions are slightly changed by an amount of adhesive or curing conditions during mounting, pressing conditions of a press plate, and the like. In general, an optical fiber often has a spare length of several tens cm. A method of holding the spare length of the optical fiber and the difference between the winding directions of optical fibers in manufacturing the optical fibers influence the variations in mounting conditions. When an optical fiber is fixed using a metal such as a solder, the thickness of the formed solder or the like and the thickness of a metal coat of the optical fiber may easily cause dispersion. Even when optical fibers are mounted on mounting holders which have almost identical processing states and the same pressing conditions, the deviation between the mounting holder is easily increased.
As described above, in the conventional mounting technique using a mechanical fitting operation, the positions of optical fibers in V-grooves have relative deviations in units of mounting holders, and the deviation between the mounting holders cannot be easily controlled to be several .mu.m or less. These relative deviations do not much influence multi-mode fibers each having a large core diameter, but easily influence single-mode fibers because each single-mode fiber has a core diameter as small as about 10 .mu.m and an allowable positional error which is strictly set to be 1 to 2 .mu.m or less. That is, absolute positional adjustment for a mounting holder cannot be easily performed at a high precision by only a mechanical assembling process.
In optical coupling between an optical semiconductor device and an optical fiber or an optical waveguide, or in an optical semiconductor module such as a so-called IC card in which semiconductor integrated circuits are mounted on a portable substrate at a high density, an input/output coupling portion must be connected to another coupling body, e.g., an optical connector or a multi-pin connector, to have a position adjusted with respect to the coupling body at a high precision because of the following reason. That is, in the former case, i.e., in the optical coupling, when a single-mode optical fiber or optical waveguide is optically coupled to a semiconductor laser, a high positional precision is required; in the later case, i.e., in the optical semiconductor module, as the storage capacity of the IC card is larger, the number of connecting pins of the IC card is increased and the interval between the connecting pins is narrowed to set the size of each pin to be small. For example, in the coupling between an optical fiber and a semiconductor laser, a multi-mode optical fiber requires a precision of .+-.5 .mu.m or less, and a single-mode optical fiber requires a precision of .+-.2 .mu.m or less. In the IC card, the required precision is gradually strict, and a precision of .+-.10 .mu.m or less at a pitch of 100 .mu.m is considered in the future.
FIG. 9 shows an optical semiconductor module obtained according to a prior art devised to meet the above request (e.g., Japanese Patent Application No. 3-238038). A submount 76 constituted such that a silicon substrate 61 and a silicon substrate 63 are directly bonded to each other using an oxide film 62 as an adhering interface is tightly interposed between two guide pins 65 and mounted on a copper stem 69. A recessed portion having a depth reaching the depth of the adhering interface is formed in the submount 76 by etching, and a semiconductor chip 64 is soldered on the bottom surface of the recessed portion.
As shown in FIG. 10, the submount 76 arranged to be tightly interposed between the two guide pins 65 is positioned and fixed by a pressure such that a press plate 75 is fixed on the copper stem 69 by tightening screws in holes 74. That is, the position of the surface of the semiconductor chip 64 in a vertical direction is determined by the sum of the thicknesses of the semiconductor chip 64 and the silicon substrate 61. The position in a horizontal direction is determined by controlling the position of the submount 76 relative to the guide pins such that the width of the submount 76 cut by dicing or the like is set to be equal to the interval between the guide pins 65. In this case, the horizontal position of the semiconductor chip 64 is determined such that the above recessed portion is arranged at a predetermined position of the submount 76, thereby arbitrarily setting the position of the submount 76 with reference to the guide pins 65.
However, in the above prior art, a vertical force generated when the submount is positioned and fixed acts on the guide pins 65 and the copper stem 69, but a lateral force disadvantageously acts on contact points between the submount 76 and the guide pins 65. For this reason, a semiconductor chip or the like which is easily broken by an external force is difficult to be directly mounted between the guide pins. Even when a submount which is relatively strong against the external force is used, a force acting in the lateral direction may break the silicon substrate, and a cutting precision must be strictly controlled to properly set the lateral dimensions. Even if a submount is cut to have a properly controlled width because the cutting precision of dicing is high, the central position of the submount 76 is often shifted with respect to the cut ends of the submount. Therefore, difficulty in manufacturing a module, the stability of a mechanical position against a change in temperature, and the like pose problems.
As has been described above, a mass-production type optical semiconductor module which can be manufactured at a low cost using no optical axis adjustment but using a highly precise module substrate which can be precisely formed in mass production by the semiconductor process, temperature characteristics and optical coupling characteristics disadvantageously have poor reproducibility.